This invention relates to eddy current instruments for the non-destructive detection of flaws in electrically conductive articles and more particularly to the detection of flaws in the regions of electrically conductive material surrounding fastener holes.
Eddy current instruments of this type utilizing a source or exciting coil for inducing an alternating magnetic field in a material and a pick-up coil for detecting the magnetic field induced are widely used for locating flaws or imperfections in metallic materials. An eddy current instrument of this type detects a flaw by detecting the metal loss due to the flaw. The localized presence of a flaw or crack in the metal upsets the otherwise normal eddy current distribution. This abnormal apportionment of eddy current creates a resultant change in magnetic flux which is sensed by a change in the inductance of the pick-up coil.
Structural wing skins on cargo aircraft are spliced to each other in an overlapping configuration such that the skins are held together by means of fasteners mounted through holes in both skins. Fatigue damage can occur at the fastener hole in either layer of skin. The disclosed apparatus is not intended to be limited to fastener hole flaw detection but also may be utilized to detect surface flaws in any metallic (ferrous or non-ferrous) structure having flat surfaces, in such applications the anomalous flaw signal would be detected by phase-amplitude deviations as in the fastener hole flaw detection application. Controlling of the size and shape (directivity) of the eddy current pattern induced in the inspected material has proved to be a most intricate probe design problem, as addressed in the Sarian U.S. Pat. No. 4,088,953.
Conventional eddy current equipment is available with cup shaped core type probes designed for the detection of large cracks emanating from fastener holes in second or interior layers. Early probe designs interrogate the entire fastener hole during inspection and consequently are not very sensitive to the presence of small cracks. Such absolute measurement systems have an inherent low resolution because the flaw area makes up only a small portion of the total eddy current area under the test probe. Another criterion for reliable flaw detection is a uniform excitation field for inducing a uniform eddy current pattern in the target material, this being pointed out in the Smith et al. U.S. Pat. No. 4,594,549. Sensing the direction of the reflected eddy current pattern has taken several approaches in prior art, usually mechanical rotation, in one approach a mechanically rotatable core portion is rotated in a circular scan around the fastener hole, as in the Harris U.S. Pat. No. 4,095,181. This design and method increased resolution, but a 360 degree mechanical rotation of the probe is required for each hole location. The Lakin U.S. Pat. No. 4,495,466 was an improvement in that the circumference of the hole is broken into several angular segments, by means of a segmented cup-like sensing face structure forming a plurality of pick-up poles. Although the Lakin patent has provided higher resolution without mechanical rotation, nevertheless in the Lakin patent flaw detection depends solely on comparing mutual pick-up voltage levels looking for discrepencies between two voltage levels, and phase difference between individual pole segments is not transduced directly in the cited patent. The Journal of Nondestructive Evaluation, Vol. 2, No. 1, 1981, by B. A. Auld, et al compares three basic eddy current probes: the absolute, the differential and the ferromagnetic resonance (FMR). The microwave frequency FMR probe is disclosed as having a uniform precession mode wherein a magnetic dipole moment is created by the excitation of a sphere of yttrium iron garnet. The above citation states "Time-varying magnetic fields created outside the sphere by this rotating magnetic dipole moment interact with a flaw and produce a flaw detection signal in the same way as the spatially fixed time-alternating equivalent magnetic dipoles representing the coil-type probes" (absolute and differential). This citation is the most pertinent prior art the applicant has found to date.
It is also stated in the above citation that mouth opening flaws were easily observed with the FMR probe but were not detected with the low frequency probes. FMR probes seem to be inherently limited to the ultra high frequencies where the skin effects limit depth penetration.
The Lakin U.S. Pat. No. 4,379,261 discloses segmented (spokes) pot core structures excited by two or three phase current to create a rotating magnetic field which scans the fastener hole. The Lakin patent contends a rotating field essentially eliminated the problems due to lift-off and provided greater sensitivity to detect cracks in the second or bottom layer of material. Although this was a step in the right direction, the rotating magnetic field generated by the Lakin patent is lacking in uniformity due to the rotating field originating in a segmented driving core structure, thus having inherent spatial inhomogeneity, due to the rotating magnetic field being generated in a core structure having a finite number of pole segments. In contrast the rotating magnetic field driving the sensing elements of the present invention is generated in a hollow toroid structure having an infinite number of pole segments e.g. the magnetic domains themselves. The relative significance of this difference is, a small crack in the test specimen may produce a very slight irregularity in the reflected eddy current pattern, and thus may be obscured by a non-uniform driving field. Hall-effect and magnetoresistive chips have been utilized as the detection element in some eddy current fastener hole probes such as the Chapman et al U.S. Pat. No. 3,450,986, and such devices may be utilized with the rotating magnetic field of the present invention to provide an improved Hall-effect or magnetoresistive effect transducer of eddy-current effects.